As an exhaust gas purification catalyst for purifying the exhaust gas of automobiles, three-way catalysts in which a precious metal is supported on an inorganic oxide are known. Three-way catalysts which can efficiently remove hydrocarbons (HC), nitrogen oxides (NOX), and carbon monoxide (CO) at the same time are widely used. Exhaust gas purification catalysts represented by three-way catalysts often contain rhodium (Rh), which is excellent in NOX purification activity.
The exhaust gas removal performance of an exhaust gas purification catalyst is most efficient at the stoichiometric air-fuel ratio (also referred to as the “stoich”) around the air-fuel ratio of 14.6. However, automobiles do not always run at an air-fuel ratio near the stoich. It is known that exhaust gas purification performance tends to deteriorate when the air-fuel ratio fluctuates under the control of an engine and the exhaust gas composition (atmosphere) changes accordingly.
In order to alleviate the atmospheric fluctuation of an exhaust gas, it is common to include a ceria component, which has a function of occluding and releasing oxygen, in the exhaust gas purification catalyst. In particular, an exhaust gas purification catalyst containing this ceria component as a ceria-zirconia composite oxide (CZ) is known.
For example, Patent Literature 1 describes an exhaust gas purification catalyst in which Rh is supported on a plurality of types of CZ particles having different particle sizes.
The technology of Patent Literature 1 attempts to prevent the peeling of a catalyst layer from a substrate by using large and small CZ particles in combination, whereby the small particles enter into gaps between the large particles and the contact area between particles as well as between the particles and the substrate increases (FIG. 1).
Conventionally, catalyst layers in exhaust gas purification catalysts as described above have been formed on a substrate which itself has no exhaust gas purification capability, for example, a honeycomb substrate made of cordierite. However, in recent years, exhaust gas purification catalysts in which a precious metal is supported on a substrate composed of inorganic oxide particles have been proposed (Patent Literature 2).